This invention relates generally to sensors for measurement of aerial blood flow parameters and more specifically to the use of such sensors to control the operation of implantable medical devices.
In a number of medical and surgical procedures, it is desirable to monitor parameters of aerial blood flow at specific locations within the aerial system, in the course of diagnostic testing or monitoring of cardiovascular system performance. Aerial blood flow parameters may also be employed to control the operation of an implantable device. For example, it is known that an inverse relationship exists between aerial blood pressure and heart rate and that the baroreceptor reflex causes changes in cardiac rate to provide short term control of aerial blood pressure. In a heart which exhibits bradycardia or disassociation between the ability of the heart to increase cardiac output in response to physical exercise, it is desirable to provide cardiac pacing in order to adjust heart rate to physiologic demand in a manner similar to the human body's natural baroreceptor reflex. An aerial blood pressure sensor, for example, may be used to control the pacing rate in such a device.
It is also shown that high ventricular tachycardias and ventricular fibrillation have marked effects on hemodynamic performance and that the comprise in hemodynamic performance may be most efficaciously measured in the aerial vascular system. Aerial blood flow parameters are recognized as being useful in this context to assist in controlling the operation of anti-tachycardia devices.
The direct measurement of aerial blood flow parameters using sensors introduced into the left ventricle or the aerial system has been proposed, but is not presently practiced in the context of conically implantable devices. The primary concern is that the introduced sensor may provoke thrombus formation and/or embolization due to the difficulty of providing a high pressure seal at the point of entry to the chamber or artery. Moreover, chronic implantation often involves the accumulation of fibrotic material on the sensors leading to possible obstruction of the artery. In addition, clots formed on the sensor may break free, raising the risk of stroke. In order to avoid these problems, the use of a perivascular transducer placed adjacent to or around the exterior vascular wall of the selected artery has been proposed. However, such a device may itself poses the risk of erosion and rupture of the artery or constriction of the artery either directly or by tissue growth around the transducer.
Nonetheless, chronically implantable sensors for measuring arterial blood flow parameters for control of implantable devices continue to be proposed, indicating the desirability of obtaining measurements of arterial blood flow parameters, in conjunction with controlling such devices. For example, U.S. Pat. Nos. 4,774,950 and 4,967,749 issued to Cohen and the articles "Perivascular Impedance Sensors for In Vivo Chronic Blood Measurement: Detection Systems for Automatic Defibrillators, Cardioverters and Blood Pressure Controllers" by Tacker et al., published in the proceedings of the 37th ACEMB, September, 1984, page 20 and "A New Implantable Arterial Blood Sensor for Detection of Ventricular Fibrillation" by Konrad et al, published in Medical Instrumentation, December, 1988, Vol. 22(6): 304-311 disclose sensors for measurement of arterial blood flow. The Cohen patents envision mounting of an indwelling pressure transducer in the left ventricle or an artery, and disclose the use of measured left ventricular or arterial blood pressure to control an implantable device such as a cardiac pacemaker or anti-tachycardia device. The Tacker and Konrad articles suggest that placing the transducer within the arterial system as suggested in the Cohen is disadvantageous, and therefore suggests pulse sensors located adjacent to the carotid and femoral arteries, employing impedance plethysmography to measure pulsatile blood flow, for use in detection of ventricular fibrillation and ventricular tachycardias leading to hemodynamic compromise.
U.S. Pat. No. 4,791,931 issued to Slate discloses a pressure transducer for measurement of blood pressure adjacent to an easily accessible artery, for use in controlling the rate of a rate responsive cardiac pacemaker. A similar system is disclosed in U.S. Pat. No. 4,899,751 issued to Cohen, which employs intravascular pressure transducers similar to those disclosed in the Cohen '749 patent cited above. The Cohen patents, the Slate patent, and the cited Konrad and Tacker articles are indicative of the types of devices with which sensors manufactured and used according to the present invention may usefully be employed, and are all incorporated herein by reference in their entireties.